Process for the preparation of zinc dithionite

ABSTRACT

Disclosed is a process for preparing zinc dithionite comprising reacting zinc metal with sulfur dioxide, wherein the zinc metal used in the reaction is produced by electrochemical reduction in an undivided electrochemical cell of an aqueous alkaline slurry or solution of zinc oxide or any other zinc compound that reacts with aqueous base to form zinc oxide.

FIELD OF THE INVENTION

[0001] The present invention provides a process for the preparation of zinc dithionite that utilizes electrochemically prepared zinc metal as a starting material.

BACKGROUND OF THE INVENTION

[0002] Zinc metal is generally produced by pyrometallurgical or electrochemical process. Zinc dust and powder are particulate forms of zinc. The terms dust and powder have been used indiscriminately to designate particulate zinc materials. The term zinc dust designates material produced by condensation of zinc vapor, whereas zinc powder indicates the product obtained by atomizing molten zinc. Zinc dusts are manufactured in various size ranges and a typical commercial dust has an average particle diameter between 4 and 8 μm. Usually, dusts are screened to be essentially free of particles coarser than 75 μm (200 mesh). The particle size distribution for commercial zinc powders typically range from 44 to 841 μm. (325-20 mesh).

[0003] In the chemical and metallurgical industries, zinc dust is used as the reducing agent, in the manufacture of hydrosulfite compounds for the textile and paper industries, and to enhance the physical properties of plastics and lubricants. Efforts have been made to prepare zinc powder by electrolyzing a solution of zinc based compounds in strong alkaline solutions.

[0004] Orszagh and V. Vass (Hungarian Journal of Industrial Chemistry, Vol. 13, pp. 287-299 (1985)) disclose an electrochemical method to regenerate zinc mud obtained during sodium dithionite production. The zinc mud obtained in sodium dithionite production, contains 50-60% water and 30-35% zinc, mist in the form of zinc hydroxide. The zinc mud was dissolved, the solution clarified, and electrolyzed, and the zinc powder obtained was used to reduce sulfur dioxide. According to Orszag and Vaas, their process requires a number of labor intensive operations such as lifting of the electrodes, removal of the zinc powder, drying, grinding and sizing, etc., that make this process economically unattractive. Removal of all insoluble material was carried out by using sedimentation and filtration. Solution of the zinc oxide in an aqueous NaOH solution was electrolyzed to prepare zinc particles. Zinc particles so produced were washed with water (20 to 60 ml water per gram of the zinc), dried, ground and sieved to give zinc particles with the desired particle size distribution

[0005] Furthermore, their process requires a divided electrolytical cell. This leads to a significant increase of operating, maintenance and capital expenses. In order to reduce the required operating, maintenance and capital expenses, it is highly desirable to develop a process which uses an undivided cell. The present invention affords such a process.

[0006] The process of Orsagh and Vass further requires high cell voltage at a lower current density. This increases capital as well as operational expenses. In summary, this process is economically unattractive as an alternate to the currently used zinc dust based technology. Because of these disadvantages, there is a strong need for an improved cost effective process to recycle zinc oxide byproduct to zinc dithionite. The present invention provides such a process.

[0007] U.S. patent application Ser. No. 09/776,518 (filed Feb. 2, 2001) discloses an electrochemical process for preparing zinc powder which involves: a) providing to an electrochemical cell a basic solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the basic solution prepared by dissolving the zinc oxide or the other zinc compound in an aqueous 2.5 to 10.0 M base solution; and b) passing current to the cell at a current density of about 10,000 to about 40,000 A/m² for a time period sufficient to electrochemically reduce the zinc oxide to zinc powder, wherein the electrochemical process has a current efficiency of at least 70% and is substantially free from electrode corrosion.

[0008] U.S. patent application Ser. No. 09/776,644 (filed Feb. 2, 2001) discloses a continuous electrochemical process for preparing zinc powder which involves: providing to an electrochemical cell a solution or suspension in an aqueous 1.25 Molar to 10.0 Molar base solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the solution or suspension containing at least 2 millimoles of solubilized zinc based species per 100 grams of electrolyte; and b) passing current to the cell at a current density of about 500 to 40,000 A/m², for a time period sufficient to electrochemically reduce the solubilized zinc based species to zinc powder, while continuously or intermittently adding a sufficient amount of the zinc oxide or the other zinc compound to the cell to maintain the concentration of the solubilized zinc based species at a level of at least 2 millimoles per 100 grams of electrolyte and continuously or intermittently removing at least a portion of the zinc powder formed; wherein the electrolyte includes the aqueous base solution and the zinc oxide or the other zinc compound.

[0009] U.S. patent application Ser. No. 09/965,157 (filed Sep. 27, 2001), provides a process for preparing a solution of zinc oxide in an aqueous base, said process comprising diluting a more concentrated solution of zinc oxide in aqueous sodium or potassium hydroxide to produce a resulting dilute solution of zinc oxide having a concentration of zinc oxide that is higher than that obtained by dissolving solid zinc oxide in aqueous sodium or potassium hydroxide, wherein the concentration of the aqueous sodium or potassium hydroxide used for dissolving the solid zinc oxide is substantially the same as the concentration of the aqueous sodium or potassium hydroxide in the resulting dilute solution of zinc oxide, and wherein the concentration of the aqueous sodium hydroxide in the resulting dilute solution ranges from 5 wt % NaOH to about 35 wt % NaOH, and the concentration of the aqueous potassium hydroxide in the resulting dilute solution ranges from 10 wt % KOH to about 55 wt % KOH.

[0010] U.S. patent application Ser. No. 10/015,185 (filed Dec. 7, 2001) provides a low corrosion electrochemical process for preparing zinc metal which comprises electrochemically reducing an aqueous basic solution or slurry of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, wherein the electrochemical process is carried out in an undivided electrochemical cell, and wherein air or nitrogen is bubbled in through the solution or slurry of zinc oxide or said other zinc compound during said electrochemical process.

SUMMARY OF THE INVENTION

[0011] The present invention provides a process for preparing zinc dithionite comprising reacting zinc metal with sulfur dioxide, wherein the zinc metal used in the reaction is produced by electrochemical reduction in an undivided electrochemical cell of an aqueous alkaline slurry or solution of zinc oxide or any other zinc compound that reacts with aqueous base to form zinc oxide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Careful washing and drying of the zinc powder produced by electrolysis of zinc based species is of major importance because the wet powder, with its large specific surface area, is quite susceptible to oxidation.

[0013] Even water oxidizes this high surface area zinc. For this reason, wet zinc powder produced electrochemically loses some reactivity during storage. For this reason, wet zinc should be used immediately. This high surface area zinc is also expected to be oxidized with oxygen and thereby lowers the current efficiency for zinc formation. It is probably for these reasons, that previous work (e.g., Orszag and coworkers, (Hungarian Journal of Industrial Chemistry, Vol. 13, pp. 287-299 (1985) and references cited therein) aimed at the development of an industrial process for the electrochemical regeneration of zinc from the zinc oxide byproduct generated during the sodium dithionite production used a divided cell to avoid the contact of the zinc produced with the oxygen generated at the anode. If an undivided cell is used, oxygen evolution from the anode introduces the possibility that it will react with the high surface area zinc and thereby will reduce the current efficiency.

[0014] The following are the highlights of the process used by I. Orszagh and V. Vass (Hungarian Journal of Industrial Chemistry, Vol. 13, pp. 287-299 (1985)) to recycle zinc oxide byproduct to zinc dithionite:

[0015] 1. Zinc oxide byproduct was dried to a zinc content of 60 to 65%.

[0016] 2. Dried zinc oxide byproduct was dissolved in an aqueous sodium hydroxide solution to give a slurry.

[0017] 3. This slurry was clarified by sedimentation and filtration. This step was necessary because these impurities are reported to contaminate the zinc powder and foul the diaphragm separating the anode from the cathode. This further suggests that the use of the slurry of zinc oxide byproduct from sodium dithionite production in an aqueous solution of sodium hydroxide may give a contaminated zinc powder and may be less reactive than the zinc dust used in manufacturing sodium dithionite. Contamination may arise because of the deposition of these impurities on the zinc produced. For example, zinc produced by electrolyzing the alkaline slurry of zinc salts in an undivided cell contains metal ion impurities generated from the anode corrosion.

[0018] 4. The zinc oxide solution was electrolyzed in a diaphragm type electrolysis unit (a divided cell). Electrolysis is carried out at a constant current density of 1000-3000 Amps/M² and this resulted in a cell voltage of 4 or more volts.

[0019] 5. Once the electrolysis was completed, the cathode was removed along with the diaphragm. The zinc powder was removed by the metal plate. The wet powder was filtered, washed, dried, milled and screened to achieve the desired particle size distribution.

[0020] 6. Zinc powder was then used in the synthesis of zinc dithionite.

[0021] As already mentioned, the present invention does not suffer from the disadvantages resulting from the method of Orszagh and Vass.

[0022] The present invention provides a process for preparing zinc dithionite comprising reacting zinc metal with sulfur dioxide, wherein the zinc metal used in the reaction is produced by electrochemical reduction in an undivided electrochemical cell of an aqueous alkaline slurry or solution of zinc oxide or other zinc compound (such as zinc sulfate, zinc acetate, and zinc carbonate) that reacts with aqueous base (alkali) to form zinc oxide.

[0023] The use of an undivided cell in the electrochemical production of zinc metal in the present invention requires lower capital. Furthermore, operational and maintenance costs are also lower when an undivided cell is used. The design of the undivided cell is simpler and the cell voltage required to achieve the desired current density is lower because of the lower ohmic resistance. This means that the electrical cost is generally lower where an undivided cell is used. Furthermore, capital cost required with the undivided cell is significantly lower than the divided cell. Details for the electrochemical production of zinc metal utilizing solutions or slurries of zinc based species in aqueous base (alkali) have been previously disclosed in U.S. patent application Ser. No. 09/776,518, 09/776,644, 09/965,157 and and 10/015,185.

[0024] In one embodiment, the aqueous alkaline slurry or solution of zinc oxide or any other zinc compound or compounds that react with the aqueous base to form zinc oxide comprises solubilized zinc based species, the species comprising at least one member selected from the group consisting of ZnO₂ ²⁻, HZnO₂ ¹⁻, Zn(OH)⁺, Zn(OH)₂, and Zn²⁺. Zinc oxide is known to dissolve by reacting with water to form a variety of species (which includes ionic and neutral species) depending upon pH. Thus a solution of zinc oxide in alkaline solution may contain species such as ZnO₂ ²⁻, HZnO₂ ¹⁻, Zn (OH)₂, Zn(OH)⁺, and Zn²⁺. Therefore, solubilized zinc based species may comprise one or more of these species in the solution.

[0025] The aqueous base solutions employed in the process of the invention are prepared by combining water with a source of alkali metal or alkaline earth metal ions, such as lithium sodium and potassium, and a source of hydroxyl (OH⁻ ions). A single source may of course provide both types of ions. The various alkali or alkaline earth metal ions are preferably supplied from various compounds such as hydroxides and oxides. Preferred base solutions are sodium and potassium hydroxide solutions.

[0026] The anode of the undivided electrochemical cell of the present invention may be made from any conventional suitable material such as platinum, or iridium, either of which may be coated over an inert support such as niobium or titanium. The anode may also be made of nickel, or from conventional materials having good alkali corrosion resistance, e.g., lead or stainless steel. The cathode may be made from any conventional suitable materials having good alkali corrosion resistance, such as magnesium, magnesium alloy, copper, lead, nickel and stainless steel. Preferably, the anode in the present invention is formed of stainless steel or nickel and the cathode is formed of stainless steel, magnesium, magnesium alloy or copper. In one embodiment, the cathode and the anode are magenesium and stainless steel respectively, and in one embodiment, magnesium and nickel respectively, and in one embodiment, copper and nickel respectively, and in one embodiment, copper and stainless steel respectively.

[0027] In one embodiment, the electrochemical reduction of the presently claimed process is conducted at a temperature of from 10° C. to 105° C., more preferably from 20° C. to 80° C.

[0028] The zinc metal prepared electrochemically is used to prepare zinc dithionite by reaction of the zinc metal with sulfur dioxide by conventional processes well known to those of ordinary skill in the art. In one embodiment, the zinc method used in the present invention is the form of wet zinc metal particles. By electrolyzing a slurry of zinc oxide by-product (from the reaction of zinc dithionite and sodium hydroxide to form sodium dithionite), wet zinc particles can be prepared (particle size distribution depending on the cathode and anode used), which can be washed and used without further processing to prepare zinc dithionite.

[0029] The process of the present invention is substantially free of labor-intensive steps such as removal of cathode after completion of the electrochemical reduction. Labor intensive and time consuming steps such as the removal of cathodes after complete electrolysis are replaced by mechanical or manual scraping of cathodes during electrolysis. The process of the present invention is also substantially free of other labor-intensive steps such as clarifying a slurry of zinc oxide in an aqueous basic solution sedimentation and filtration prior to the electrochemical reduction. Such labor intensive steps are set forth in the Orszag and Vass reference. Also, as set forth above, the present invention employs less costly undivided cell for preparing zinc metal and it is found that such zinc metal is adequately suitable for the preparation of zinc dithionite. Furthermore, it has been unexpectedly found that wet zinc can be used without the labor-intensive steps of drying, grinding and sizing of zinc metal particles prior to the reaction of zinc metal with sulfur dioxide to prepare zinc dithionite.

[0030] Current efficiency (C.E.) for the presently claimed process for the formation of zinc dithionite can be defined as follows:

C.E. for the formation of ZnS₂O₄=[(Moles of ZnS₂O₄)/(Moles of electrons passed/2)]×100.

[0031] Current efficiency for the formation of zinc dithionite can also be obtained by multiplying current efficiency for the zinc formation with conversion efficiency of zinc to zinc dithionite. Current efficiency of zinc formation is quite high (such as>90%). If reactivity of the zinc formed is similar to the zinc dust currently used in manufacturing zinc dithionite, conversion efficiency of zinc to zinc dithionite is also quite high. This then suggests that the current efficiency for the zinc dithionite formation should also be quite high. In one embodiment, the current efficiency for the formation of zinc dithionite is at least 70%, and in one embodiment, at least 75%. Experimental current efficiencies for the zinc dithionite formation in an undivided cell varied from 73% to 85% depending on several variables such as the electrodes used, current density used (such as 500 to 20,000 Amps/m², preferably 2500 to 7500 Amps/m², and more preferably 4000 to 6000 Amps/m²), concentration of sodium hydroxide used to dissolve zinc oxide byproduct, etc. This also depends on the amount and nature of impurities in the zinc oxide byproduct. These results clearly suggest that the presently claimed process (using an undivided cell) is highly effective and advantageous for the preparation of zinc dithionite from the zinc oxide by-product from sodium dithionite production.

[0032] In one embodiment of the present invention, the reaction of zinc metal with sulfur dioxide to produce zinc dithionite is conducted in the presence of cadmium sulfate, and in one embodiment in the substantial absence of cadmium sulfate. While not wishing to be bound by theory, it is believed that the addition of cadmium sulfate has a positive effect on the production of zinc dithionite, in that the yield of zinc dithionite is slightly improved. However, the reaction can also be carried out in the substantial absence of cadmium sulfate (e.g., where cadmium sulfate is not added to the reaction mixture), with good results.

[0033] The following specific examples will provide detailed illustrations of the methods of producing and utilizing compositions of the present invention. These examples are not intended, however, to limit or restrict the scope of the invention in any way and should not be construed as providing conditions, parameters or values which must be utilized exclusively in order to practice the present invention. Unless otherwise specified, all parts and percents are by weight.

EXAMPLES Example 1

[0034] General procedure:

[0035] In these experiments, a resin Kettle (5 inch (12.7 cm) in diameter and 18 inch (45.7 cm) high) was used as the cell. A solution or slurry of zinc oxide in the aqueous sodium hydroxide solution (3 to 3.5 liters) at 20 to 80° C. was charged into the resin kettle. A thermometer, desired cathodes and anodes were positioned in the cell using laboratory clamps. Mixing was achieved by using a mechanical stirrer. In some experiments bubbling nitrogen in addition to mechanical stirring was used. Parts of the cathode and anode surfaces were covered with Teflon or electrical tape to achieve the desired active cathode and anode surface areas. Electrolysis was carried out at a current density of about 5000 to 20,000 Amps/m². A portion of the zinc deposited on the cathode was mechanically removed periodically and collected on the bottom of the kettle. At the end of the experiment, zinc particles were separated from the electrolyte by decantation, and washed with water. Wet zinc (A grams) was charged into a one-liter resin kettle equipped with a thermometer, nitrogen inlet and outlet adapter, a gas disparager, mass flow meter to measure the amount of sulfur dioxide passed, a pH probe, and a mechanical stirrer. An amount (X ml of 0.18%) of an aqueous solution of cadmium sulfate was then charged into the resin kettle. The step of adding the cadmium sulfate can, however, be eliminated. The reaction mixture was then heated to 430C and sulfur dioxide gas was passed through the reaction mixture. The sulfur dioxide addition rate was controlled to maintain pH of the reaction mixture above 3.7 and temperature was maintained at 43 to 55° C. Initial addition rate of sulfur dioxide was 3.4 g/minute. Zinc dithionite solution was decanted to give a translucent solution of the zinc dithionite. It was then titrated to determine the concentration of zinc dithionite.

Example 2

[0036] The results of electrolysis of zinc oxide followed by the reaction with sulfur dioxide under various conditions are shown below in Table 1. TABLE 1 ZnO¹ Mechanical Zn₂S₂O₄ Exp. NaOH Source Stirring/N₂ A X Soln/ Cathode/ Moles e⁻¹ Moles C.E. conc. C.D.⁵ No. (wt %) (wt %) passed g ml Slurry Anode passed Zn₂S₂O₄ (%) (g/l) (A/m²) 1 15 F/7.25 Yes/No 920. 0 Soln² Mg/SS 6.02 2.37 79 459 20,000 2³ 25 W/6.0 Yes/yes 926 8 slurry Mg/Ni 6.32 2.62 83 550 5000 3 25 W/6.0 Yes/Yes 926 8 slurry Cu/Ni 6.32 2.4 76 500 5000 4 25 W/6.0 Yes/Yes 926 8 soln Cu/Ni 6.32 2.4 76 503 5000 5⁴ — — — 926 8 — — — 2.68 85 605 — 6⁴ — — — 926 8 — — — 2.63 83 553 —

[0037] Each of the documents referred to above is incorporated herein by reference in its entirety, for all purposes. Except in the Examples, or where otherwise explicitly indicated, all numerical quantities in this description specifying amounts and concentrations of materials, reaction and process conditions (such as temperature, current density, current efficiency), and the like are to be understood to be modified by the word “about”.

[0038] It must be noted that as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. Unless defined otherwise all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs.

[0039] While the invention has been explained in relation to its preferred embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims. 

What is claimed is:
 1. A process for preparing zinc dithionite comprising reacting zinc metal with sulfur dioxide, wherein the zinc metal used in the reaction is produced by electrochemical reduction in an undivided electrochemical cell of an aqueous alkaline slurry or solution of zinc oxide or any other zinc compound that reacts with aqueous base to form zinc oxide.
 2. The process of claim 1, wherein the aqueous alkaline slurry or solution of zinc oxide or any other zinc compound that reacts with the aqueous base to form zinc oxide, comprises solubilized zinc based species, said species comprising at least one member selected from the group consisting of ZnO₂ ²⁻, HZnO₂ ¹⁻, Zn(OH)⁺, Zn(OH)₂, and Zn²⁺.
 3. The process of claim 1, that is substantially free of labor-intensive steps comprising removal of cathode after completion of the electrochemical reduction of the zinc oxide.
 4. The process of claim 1, that is substantially free of labor-intensive steps comprising clarifying a slurry of zinc oxide in an aqueous basic solution by sedimentation and filtration prior to the electrochemical reduction of the zinc oxide.
 5. The process of claim 1, that is substantially free of labor-intensive steps comprising drying, grinding, and sizing of zinc metal particles prior to the reaction of the zinc metal with sulfur dioxide.
 6. The process of claim 1, wherein the zinc metal used for reaction with sulfur dioxide is in the form of wet zinc metal particles.
 7. The process of claim 1, wherein the current efficiency for the production of zinc dithionite is at least 70%, the current efficiency for said production being obtained by multiplying the current efficiency for zinc metal formation with the conversion efficiency of zinc metal to zinc dithionite.
 8. The process of claim 7, wherein the current efficiency for the production of zinc dithionite is at least 75%.
 9. The process of claim 1, wherein the undivided electrochemical cell utilizes a magnesium or copper cathode.
 10. The process of claim 1, wherein the undivided electrochemical cell utilizes a stainless steel or nickel anode.
 11. The process of claim 1, wherein the undivided electrochemical cell utilizes a magnesium cathode and a stainless steel anode.
 12. The process of claim 1, wherein the undivided electrochemical cell utilizes a magnesium cathode and a nickel anode.
 13. The process of claim 1, wherein the undivided electrochemical cell utilizes a copper cathode and a nickel anode.
 14. The process of claim 1, wherein the undivided electrochemical cell utilizes a copper cathode and a stainless steel anode.
 15. The process of claim 1, wherein the electrochemical reduction is carried out at a temperature range of about 100 to about 105° C.
 16. The process of claim 14, wherein the aqueous base comprises ions of at least one alkali or alkaline earth metal and hydroxyl (OH⁻) ions.
 17. The process of claim 15, wherein the alkali and alkaline earth metal ions are selected from sodium, potassium, and mixtures thereof and are provided in the form of a compound selected from hydroxides, and oxides.
 18. The process of claim 15, wherein the compound is selected from the group consisting of sodium hydroxide and potassium hydroxide.
 19. The process of claim 1, wherein the electrochemical reduction is carried out at a current density of about 500 to about 30,000 Amps/m².
 20. The process of claim 1, wherein the reaction of the zinc metal produced electrochemically in an undivided cell with sulfur dioxide is carried out at a temperature of about 20 to about 60° C.
 21. The process of claim 1, wherein the reaction of zinc metal with sulfur dioxide to produce zinc dithionite is conducted in the presence of cadmium sulfate.
 22. The process of claim 1, wherein the reaction of zinc metal with sulfur dioxide to produce zinc dithionite is conducted in the substantial absence of cadmium sulfate. 